A Switching Mode Power Supply
1.0 Scope
The chapter provides service information for a 22 Watts single output (12V), single phase input (120 or 240 VAC) switching-mode power supply (S.M.P.S.).
2.0 General Specifications
2.1 AC operation voltage I/P : 120VAC (90 - 130VAC) or 240VAC
2.2 AC operation frequency I/P
2.3 DC O/P regulation
2.4 DC O/P current
12 ± 1% VDC for Vol ; ± 5% for Vo2 1.2 - 1.5A for Vol ; 300 mA for Vo2
3.0 Theory of operation
Reference to Figure 1, S.M.P.S. block diagram and Figure circuit diagram.
3.1 Block diagram description
The AC Line is connected to rectifier (D701 - D704) through the line filter (L701, C701 - C704), when the power switch (SW700) is turned on. The rectifier and DC filter circuit produce about 300 VDC from the AC Line. After the input capacitors (C710, C712) are charged, the start circuit (R705, C716 etc) make the switching circuit (Q701) and the power transformer (T701) operate. The power transformer converts the 300 VDC to 35.714KHz (Horizontal Frequency) oscilation square wave and transfers the high voltage to appropriate level for the output voltage which is produced by half wave rectification and LC filter (D712, C722, C723, L702 etc). The outputs are 12 VDC which are sensed and regulated by the power transformer that control duty cycle modulator (ZD702, Q702 - Q704 etc) and drive circuit (C716, R706 etc).
4.0 Circuit description
The following paragraphs describe each block of the Block Diagram
4.1 Rectifier and DC filter (120 VAC operation)
When the power supply is connected for 120 VAC line operation the jumper wire J701 should be short. That the rectifier (D701 - D704) is connected as doubler producting 300 VDC.
4.2 Rectifier and DC filter (240 VAC operation)
When the power supply is connected for 240 VAC, the jumper wire J701 should be open. That the rectifier circuit is used as a bridge rather than a doubler as in the 120 VAC connection. In this case, the rectifier output is also approximate 300 VDC.
4.3 Start circuit, Switching circuit, Power transformer
When the rectifier output is reaching 300 VDC, the start circuit make the switching circuit operation, then the power transformer converts 300 VDC to 35.714 KHz square wave and transfers it to the appropriate level for the output.
4.4 Duty cycle modulator & drive circuit
Duty cycle modulator provide a reference voltage DC 6.2V (ZD709) to compare with output voltage sensor (12V output voltage adjustment VR701) that control the feedback voltage of the transformer, so duty cycle modulation and switching operation will be completed.
4.5 Isolation transformer (T702)
It makes the oscillation frequency of the switching circuit synchronize with the horizontal frequency.
B. VIDEO CIRCUIT
The video signal is applied to the input connector cable. The IC 401 SN7406 is utilized as video buffer/former circuit. The video amplifier gain is decided by R407, and load resistor R415. The peaking coil L401 is compensated for video frequency response. D403 is a protection diode.
C. CRT CIRCUIT
High voltage is applied to the CRT bulb to provide anode voltage. "CAUTION". This is very high voltage, over several thousand volts. D311 rectified the pulse from flyback then filtered by C406 to maintain at a normal 40 Volts is used for Q402 collector. Grid G2 is the acceleration grid and is maintained at a normal 600 Volts rectified by D309 and C318, G4 is the focus grid. VR304 adjust the focus voltage from —100 V to 600V to improve focus.
D. VERTICAL CIRCUIT
3.1 The vertical IC TDA1170N is a monolithic integrated circuit in a 12 — lead quad in-line plastic package. The functions incorporated are;
1) Synchronization circuit
2) Oscillator and ramp generator
3) High power gain amplifier
4) Flyback generator
5) Voltage regulator
3.2 Oscillator circuit
The vertical sync pulses are coupled through C201 to pin 8 of IC201, VR201 adjusts the frequency of the vertical oscillator. The D202, C210 is made up of pump up voltage.
3.3 Vertical output
The vertical sweep output is obtained at pin 4, 10 and 11 of IC201. R212, C205, R211 are made up of DC feedback of amplifier. R209, R210 are AC feedback.
The vertical size of the raster or amplitude of the sweep is adjusted by VR202. The linearity of the vertical sweep is adjusted by VR203.
E. HORIZONTAL CIRCUIT
The IC301 SN74LS221 is a dual, monolithic, non-retriggerable, high-stability one shot. The output pulse width, tw can be varied over 9 decades of timing by proper selection of the external timing components, C303 and R304, C304 and R303. Pulse width is defined by the relationship:
The flyback transformer generates the high voltage necessary for the anode voltage of the CRT. A secondary winding supplied G1; G2. The deflection coil current source is also obtained from the output of Q302. The amplitude of the pulse is approximately 190Vp-p, L302 adjust horizontal width, and L303 controls the horizontal linearity. C307 is a S-shaping capacitor.
F. HIGH VOLTAGE SUPPLY CIRCUIT
High voltage is obtained by rectification of the high voltage output of the flyback transformer T301. This diode is molded into the transformer housing.
G. AUDIO CIRCUIT
The IC50i is an audio amplifier. The power supply is provided from pin 6. Pin 4 is the ground lead. The input audio signal is coupled from pin 3 and after amplified the output signal is from pin 5 to the speaker, VR501 is volume adjustment. C505 is a compensation capacitor to avoid from high frequency oscillation. R504, C504 can increase the high frequency loading and keep from the high-frequency oscillation. R502 is a voltage gain resistor. C502 is a DC block capacitor. R503, C507 are the impedance matching components R501 is
the bias resistor. C501, C510 and C506 are the signal coupling capacitor. C508 is the filter capacitor.
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